It is currently believed that the mechanism of Ca2+ release from the sarcoplasmic reticulum (SR) operates, in skeletal muscle, under control of the membrane potential of the transverse tubular system. With the aid of electrophysiological and optical techniques, it is proposed to investigate the regulation of the intracellular Ca2+ concentration at two levels: a) the mechanisms by which the SR may initiate the changes in myoplasmic [Ca2+] during the activation of the release process and b) the mechanisms of propagation of the localized Ca2+ release SR to the rest of the myoplasm. The experiments aim to obtain precise information about critical questions that can be raised about these processes: Does the SR releases Ca2+ by means of a large or a small permeability? What is the role of the calcium-induce-calcium release process? Is the Ca2+ release process localized at the Z lines in frog skeletal muscle fibers? Me there steep intra-sarcomeric (ca2+i gradients that dissipate rapidly by diffusion of Ca2+ to the myofibrillar space? The answers to several of these questions have been pending in the field since decades. Nevertheless, technological advances in fast flash photolysis, Ca2+ detection, and imaging techniques make it possible for them to be answered experimentally now. The results may lead to the development of a new picture in the way that we understand muscle physiology today. It is conceivable that the cure for muscle diseases not well understood yet due to the lack of an accurate definition of the problems involved in the regulation of intracellular Ca2+, may be reached in view of these results. Finally, since this application deals with general questions about intracellular Ca2+ regulation, the conclusions will not only apply to skeletal, cardiac, and smooth muscle, but to almost any cell, since cytosolic Ca2+ mediates a large variety of cellular processes in biology.